The present invention relates to PCM signaling and reframing arrangements in digital telephone communication systems using frame-organized data or speech transmission. The conventional D2 channel-bank pattern employs the 193rd bit of odd frames to provide a repeating pattern 1010. . . for framing synchronization. The 193rd bit of even frames is utilized to provide a repeating pattern 000111. . . for identification by 01 and 10 transitions of the sixth and twelfth frames. The eighth bit of each channel may then be used for supervisory signaling related to the respective channel.
This conventional pattern does not provide for common-channel signaling in combination with D2 signaling over the same T1 line. Such would be very desirable for signaling between a digital central office and a remote line switch for many reasons, for example the following:
(1) Command information from a remote line switch to a digital central office may be necessary for functions such as (a) reporting line new seizures which the line switch has detected by scanning, (b) reporting faults detected by the line switch, and (c) reporting the results of a party test.
(2) Command information from a digital central office to a remote line switch may be necessary for functions such as (a) requesting assignment of a line to a channel of the T1 line, (b) requesting release of a line from a channel of the T1 line, (c) requesting ringing to a line, including the appropriate ringing code, (d) requesting testing of a line, (e) requesting coin control in the case of a coin telephone line, (f) requesting maintenance functions to be performed, and (g) requesting a party test to identify the party originating a call on a multiparty line.
(3) Command information may also be required between a controller at the digital central office and a controller at the remote line switch. A common-channel signaling method would be a desirable approach to provide a dedicated communication link between the two control units.
(4) In addition to command signaling, it may be necessary to provide supervisory signaling between the remote line switch and the digital central office for lines that have been assigned a channel. This includes sending of dial pulses, hook flashes, and disconnect signals between a subscriber line circuit and the digital central office. In certain cases, it is also desirable to send supervisory information from the digital central office to special line circuits. For example, in certain cases it is necessary to return answer supervision to the line circuit and cause a battery reversal on the subscriber line. D2 signaling presents the preferred method for communicating supervisory information and dial pulse information between the remote line switch and the digital central office. With this approach, supervisory and dial pulse signaling from subscriber lines appear at the central office to be very similar to trunk signaling. Thus, much of the pulse receiving and supervisory logic at the central office can then be shared between handling of lines and trunks.
Using D2 signaling for supervisory and dial pulsing information presents problems in that it has previously not been feasible to combine common-channel and D2 signaling on a T1 line. Some possible solutions to this problem that have previously been suggested include the following:
(1) Use only common-channel signaling. With this approach, the logic in the remote line switch is considerably increased, since the remote line switch must perform dial-pulse counting, flash detection, and disconnect timing. Also, this approach is rather inflexible for future possible additions, such as incerting trunk circuits in a remote line switch. If D2 signaling is provided on the T1 line, the signaling facilities are fully available to provide the necessary supervisory signaling to these trunk circuits controlled by the central office logic. However, if only common-channel signaling is available, the remote line-switch logic must further be increased to provide the necessary trunk signaling such as winks, delayed dials, etc.
(2) Use D2 signaling along with one of the 24 speech channels as a common channel. This approach is undesirable since it reduces the traffic handling capacity of the T1 line to 23 channels.
Following are brief comments on three prior-art U.S. patents that are illustrative of the present state of the art:
U.S. Pat. No. 3,541,456. This patent shows a fast reframing arrangement for reframing on a repeating 10 or 01 logic pattern. Every bit in a frame is examined to find one and only one which becomes the framing candidate without following this with a confidence check. This patent does not teach the concept of sharing normal mode violation detection circuitry with reframing circuitry. This circuit does not teach combined common-channel and D2 signaling. This circuit does not employ a ROM-controlled sequential machine nor RAM storage.
U.S. Pat. No. 3,940,563. This patent discloses a method for reframing on a digital logic pattern comprising a combination of repeating 010101 and 000111 patterns on a T1 line from a D2 channel bank to reduce the probability of falsely framing on a 2 KHz tone. This patent does not examine all bits of a frame but looks for one and only one framing candidate. Hence, it might reframe falsely on a spurious candidate. This method does not provide combined common-channel and D2 signaling. This patent does not teach the use of a ROM-controlled sequential machine nor RAM storage of all framing bit candidates.
U.S. Pat. No. 4,016,368. This patent, granted Apr. 5, 1977, is assigned to the same assignee as the present invention. It discloses a ROM-controlled sequential machine providing normal mode-violation detection, with guards against transient triggering, and search mode for reframing. However, this disclosure does not employ RAM storage for all framing bit candidates. and does not teach the examination of all bits of a frame for one and only one framing candidate. Hence, it does not provide as high a degree of protection against reframing on a spurious candidate as the present invention. Neither does this system provide for combined common-channel and D2 signaling.